Magnetrons are specialized electron tubes characterized by the interaction of electrons within an electric field of a circuit element in crossed, steady electric and magnetic fields to produce an a-c power output. The design of the magnetron determines the frequency of its output of microwave energy.
In some prior art magnetrons, it is desirable to adjust the output frequency over a wide range, such as, 10 to 500 MHz. In this type of magnetron, adjustment is often accomplished by adjusting the size of the cavity by moving one of the cavity walls. See, for example, U.S. Pat. No. 3,119,082 which issued Jan. 21, 1964 by M. W. St. Clair et al.
Not all magnetrons require adjustment over such a wide range. In many cases, it is desirable to operate the magnetron at a single, fixed output frequency. In these situations, a capacitance-, or inductance-tuning device may be introduced into the crossed-field interaction space of the magnetron cavity defined between the cathode and anode thereof. An adjustable cavity is not desirable as such cavities add to the cost of the magnetron.
Where a single output frequency is desired, it is theoretically possible to design a magnetron that requires no adjustment. However, manufacturing tolerances, temperature sensitivity, and other factors require some tuning to adjust the magnetron to the desired output frequency. Other magnetrons are useful with an output which may vary within a narrow range. These magnetrons require no adjustment at all.
An example of a inductance tuning device used in the prior art to place conductive strips or rods between vanes that form the anode of a magnetron is found in U.S. Pat. No. 3,366,833 which issued Jan. 30, 1968 by P. R. Hanson. This patent illustrates a typical vane-and-strap magnetron.
An example of a capacitance tuning device which places conductive plates in, or adjacent to, the interaction space may be found in U.S. Pat. No. 3,600,629 by P. Fenster, et al. which issued Aug. 17, 1971. An example of capacitance tuning in which the conductive plates are remote from the interaction space is found in U.S. Pat. No. 3,379,925 by R. E. Edwards, issued Apr. 23, 1968.
Another prior art magnetron is the coaxial magnetron in which the cathode and anode are coaxially arranged and beyond which a cavity is arranged coaxially about the anode to form an anode cavity. Within the anode cavity is located a rotational tuning device disposed between the anode and an output of the magnetron device. The tuning device consists of a flat, paddle-like ceramic element which may be rotated to expose its edge to the microwave energy within the cavity or to expose one of two flat surfaces for adjusting the output frequency of the magnetron as shown by the curve of FIG. 5b. The rotational arrangement of the flag tuner creates some problems. The anode and cathode must be retained within a vacuum. Yet, a rotating element passing through the wall of a vacuum chamber is difficult to seal. In the prior art coaxial magnetron, this sealing problem was overcome by placing a ceramic sleeve about the anode and cathode through which the electrons could pass but which would retain a vacuum in the area of the interaction space. The outer chamber thus formed was then filled with a suitable gas which could be retained under a pressure and which could be more easily sealed from escape through the rotational flag tuner. Such a magnetron is referred to as a sleeve tube magnetron which may be purchased from the Electron Tube Division of Litton Systems, Inc., the assignee herein.
The sleeve tube magnetron requires two chambers, a vacuum chamber and a pressure chamber with a ceramic separation therebetween. Further, the adjustment of the sleeve tube magnetron is very sensitive in that the flag only rotates 90.degree. or one-quarter turn from the minimum output frequency of the magnetron to its maximum output frequency. This makes it difficult to control adjustment of the output frequency.